Filenames of the 4 images with slight corruption; during acquisition there was an imbalance in the intensities of the odd and even fields in each frame:

yaleB02_P00A+025E+00.pgm
yaleB02_P00A+020E-10.pgm
yaleB05_P00A+035E+15.pgm
yaleB05_P00A+035E+40.pgm